The present embodiments relate to wireless communication systems and, more particularly, to multi-input multi-output (MIMO) transmissions. With Orthogonal Frequency Division Multiplexing (OFDM), multiple symbols are transmitted on multiple carriers that are spaced apart to provide orthogonality. An OFDM modulator typically takes data symbols into a serial-to-parallel converter, and the output of the serial-to-parallel converter is considered as frequency domain data symbols. The frequency domain tones at either edge of the band may be set to zero and are called guard tones. These guard tones allow the OFDM signal to fit into an appropriate spectral mask. Some of the frequency domain tones are set to values which will be known at the receiver. Among these are Channel State Information Reference Signals (CSI-RS) and Dedicated or Demodulation Reference Signals (DMRS). These reference signals are useful for channel estimation at the receiver.
The past three decades have witnessed the tremendous success and growth of cellular wireless communication. The number of cell phone users has exploded during that period. This was driven in part by demand for smart phone devices that provide high-speed data services, such as video streaming, online gaming, and such. Motivated by an increasing demand for network capacity and data speed, the latest fourth generation (4G) cellular communication systems featuring 3GPP Long-Term Evolution (LTE) and LTE-Advanced (LTE-A) achieve downlink spectral efficiency up to 30 bit/s/Hz, and maximum data rates of up to 30 Gbits/s over a 100 MHz system bandwidth.
Multiple-antenna systems are one of the most important techniques used in improving the data rate of a cellular communication system. By deploying multiple transmit antennas at a base station (e.g., an evolved NodeB or “eNB” in LTE systems), the base station is able to transmit multiple data streams simultaneously over the same spectrum bandwidth, thereby significantly increasing the efficiency of spectrum usage. Multiple data streams can only be decoded by a mobile terminal (e.g., user equipment or “UE” in LTE systems) that is equipped with multiple receive antennas. Assuming there are a total of Nt transmit antennas (i.e., multiple-input) at the eNB and Nr receive antennas at the UE (i.e., multiple-output), the number of data streams r transmitted in the downlink may vary from 1 to min(Nt, Nr), which is denoted by the channel rank hereinafter. Rank adaptation is performed at the eNB by intelligent eNB scheduling, facilitated by knowledge of channel state information (CSI) of the downlink propagation channel. CSI is measured by the UE and reported to the eNB in an uplink feedback mechanism.